The actual invention relates to a method for determining an actual input value of an input variable for a control unit of an optical imaging device and to a method for driving an active component of the imaging device as a function of the actual input value. The invention can be used in connection with the microlithography used in the production of microelectronic circuits. It furthermore relates to an optical imaging method which can be carried out, inter alia, via an optical imaging device according to the invention.
Particularly in the field of microlithography, besides using components embodied with the highest possible precision, it is necessary, inter alia, to keep the position and geometry of the components of the imaging device, that is to say for example the optical elements such as lens elements, mirrors or gratings, as far as possible unchanged during operation, in order to achieve a correspondingly high imaging quality. The high accuracy requirements, which are in the microscopic range of the order of magnitude of a few nanometers or less, are in this case not least a consequence of the constant need to increase the resolution of the optical systems used in the production of microelectronic circuits, in order to advance the miniaturization of the microelectronic circuits to be produced.
In order to achieve an increased resolution, either the wavelength of the light used can be reduced, as is the case for systems which operate in the extreme UV range (EUV) with operating wavelengths in the range of 13 nm, or the numerical aperture of the projection system can be increased. One possibility for appreciably increasing the numerical aperture above the value of one is realized via so-called immersion systems, wherein an immersion medium having a refractive index of greater than one is situated between the last optical element of the projection system and the substrate that is intended to be exposed. A further increase in the numerical aperture is possible with optical elements having a particularly high refractive index.
Both with the reduction of the operating wavelength and with the increase in the numerical aperture, there is an increase not just in the requirements made regarding the positioning accuracy and dimensional stability of the optical elements used throughout the course of operation. There is also an increase, of course, in the requirements with regard to minimizing the imaging errors of the entire optical arrangement.
What is of particular importance in this case, is, of course, the temperature distribution within the components used, in particular within the optical elements, and the possibly resultant deformation of the relevant components, for example of an optical element, and a possible temperature-dictated variation of the refractive index of the relevant optical element.
For an EUV system, it is known from EP 1 477 853 A2 (Sakamoto; the disclosure of which is incorporated herein by reference) to actively counteract the heating of a mirror exclusively usable in such systems, the heating resulting from the incident light, and to actively keep a temperature detected at a specific location in the mirror within specific predefined limits. This is done via a temperature adjustment device which is arranged centrally on the rear side of the mirror and which comprises Peltier elements or the like. This solution has the disadvantage firstly that it is not suitable for use with refractive optical elements, such as are used in particular in the case of the immersion systems mentioned, since the central temperature adjustment device would cover the optically used region. Secondly, only the temperature of a single location in the mirror is reliably controlled taking account of the light energy absorbed by the mirror in a more or less stationary state. Further thermal environmental influences, in particular non-stationary and/or locally varying thermal influences, such as can be introduced by an immersion medium and which can cause dynamic or local fluctuations in the temperature distribution in the mirror, are disregarded.
Proceeding from these problems, WO 2007/128835 A1 (Gellrich et al.; the disclosure of which is incorporated by reference herein) proposes, inter alia, using thermal models of the relevant optical elements. In this regard, by way of example, via such a thermal model of an optical element, depending on the actual values of a wide variety of influencing variables (such as, for example, the light power actually used, etc.) and/or detection variables (such as, for example, temperatures measured at specific points in the region of the optical element), it is possible to estimate the actual temperature distribution in the optical element. The insights thus gained regarding the temperature distribution in the optical element can then be used as input variables for a control of the imaging device, which drives active components (for example heating elements and/or cooling elements) as a function of the input variables, in order to achieve a desired temperature distribution in the relevant optical element.
What is problematic here is that, firstly, specific influencing variables that influence the temperature distribution, such as, for example, the actual local light power, can be specified only with limited accuracy, while thermal disturbances often cannot be detected at all. This can have the effect that the estimation of the temperature distribution obtained via the model and the actual temperature distribution in the optical element deviate from one another to a greater or lesser extent, and possibly even drift further and further apart over time, with the result that it is no longer possible to control the temperature distribution as required.
This circumstance could eventually be counteracted via a corresponding refinement of the model, in particular taking into account further influencing variables and/or a larger number of detection points (at which detection variables, such as the temperature, for example, are ascertained). In this case, however, firstly the complexity for creating the thermal model would increase considerably. Furthermore, the calculation effort for ascertaining the input variables of the control and thus the expenditure of time for the driving of the active components would also increase as a result, such that the high dynamic range of the control, especially required in the field of microlithography, may possibly no longer be guaranteed.